Update comment for VG_(am_find_nsegment) to match the code.
Likewise for VG_(am_next_nsegment). Also make the code similar to
VG_(am_find_nsegment) for easier comprehension.
git-svn-id: svn://svn.valgrind.org/valgrind/trunk@14937 a5019735-40e9-0310-863c-91ae7b9d1cf9